An organic EL device in the simplest structure is generally constituted of a light-emitting layer and a pair of counter electrodes sandwiching the light-emitting layer and functions by utilizing the following phenomenon. Upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer; the energy level after the recombination returns from the conduction band to the valence band with release of energy in the form of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer composed of an aromatic diamine and a light-emitting/electron-transporting layer composed of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescent materials in place of fluorescent materials is investigated. The aforementioned device comprising a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state, is expected to enhance the luminous efficiency approximately three to four times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer has been investigated, but these derivatives merely produced luminance at an extremely low level. Thereafter, europium complexes were tried in utilization of the excited triplet state, but they too failed to emit light at high efficiency. The studies on utilization of phosphorescence are mostly centered on the use of organic metal complexes such as the iridium complexes mentioned in patent document 1 as phosphorescent dopants.    Patent document 1: JP 2003-515897 A    Patent document 2: JP Hei 5-214333 A    Patent document 3: JP Hei 11-162650 A    Patent document 4: JP Hei 11-176578 A    Patent document 5: JP 2005-093425 A
Further, studies are under way on enhancement of the efficiency and elongation of lifetime by means of lowering the driving voltage of the device. For example, patent document 2 discloses the use of quinolinol-based metal complexes such as Alq3. Even so, organic EL devices using such compounds require high driving voltage and show a short lifetime and there is a growing demand for electron-transporting materials capable of lowering the driving voltage, enhancing the efficiency, and elongating the lifetime still further.
Further, patent documents 3 and 4 disclose indolocarbazole compounds, but the disclosure is concerned with compounds in which the indolocarbazole skeleton is not linked to an aromatic heterocyclic group. Moreover, the documents recommend the disclosed indolocarbazole compounds for use as hole-transporting materials and commend them for their stability, but the documents do not teach at all the use of the disclosed indolocarbazole compounds as electron-transporting materials.
Further, patent document 5 discloses a scheme for improvement of the characteristics of the device by dividing an electron-transporting layer into a first electron-transporting layer and a second electron-transporting layer and using a specified compound in each layer. According to the disclosure, the intended effect can be manifested provided that the ionization potential of the first electron-transporting layer (IP1) is related to the ionization potential of the second electron-transporting layer (IP2) as IP1<IP2.